1. Field of the Invention
This invention relates to an orthodontic spreader, which comprises an actuating screw and pressure-transmitting nuts screwed on both end portions of the screw and is mainly intended for correcting the positions of molars in the upper jaw.
2. Description of Prior Art
Such orthodontic spreaders which comprise an actuating screw and pressure-transmitting nuts screwed on both end portions of the screw and are mainly intended for correcting the positions of molars in the upper jaw are known, in which said actuating screw has end portions provided with oppositely handed screw threads and is operable to move said pressure-transmitting nuts along parallel guide rods. For instance, one known jaw-expanding spreader comprises two housing members, which are adjustable by means of an actuating screw and are guided by a single pin or by two pins. Each guide pin is an interference fit in one housing member and a sliding fit in the other housing member. In that arrangement the force-fitted guide pins increase the shortest length to which the spreader can be contracted. In order to reduce that shortest length, orthodontic spreaders have been proposed, wherein a spreader can be contracted to a shorter length because a guide extension extends from the screw-threaded portion of the first housing member toward the second housing member so that the length required for the fixation of the guide pins is reduced.